An offset lithographic printing press typically comprises a plate cylinder, an ink form roll and a dampener form roll supported for rotation in the press. The plate cylinder carries a printing plate having a surface defining an image to be printed. The surface of the printing plate has portions which are treated to receive ink and reject dampening liquid, and other portions which are treated to receive dampening liquid and reject ink.
Typically, a succession of inker rolls transfers ink from an ink fountain to the ink form roll. The ink form roll applies a film of the ink to the printing plate surface at a nip between the plate cylinder and the ink form roll. A succession of dampener rolls transfers the dampening liquid from a dampening liquid fountain to the dampener form roll. The dampener form roll applies a film of the dampening liquid to the printing plate surface at a nip between the plate cylinder and the dampener form roll. The press also includes a blanket cylinder which picks up the inked image from the printing plate surface at a nip between the plate cylinder and the blanket cylinder. The blanket cylinder transfers the inked image from the printing plate to the material being printed, such as a moving paper web.
When the ink form roll applies ink to the printing plate surface, it encounters both ink and dampening liquid at the printing plate surface Dampening liquid applied to the printing plate surface by the dampener form roll may be picked up by the ink form roll. Such dampening liquid picked up by the ink form roll can interfere with the application of ink to the material being printed A variety of print defects can occur, such as mottle, stripes, spots, and the like.